Best Place for Learning FRC Motor Mechanics?

#1

Basically, our team has a decent handling on how to program motors, but little knowledge on the best way to properly utilize them mechanically. Even though our robot is a cooperative effort, each member of our small team usually specializes in different functions of the robot, but most of us have little knowledge when it comes to motor accessories and how to utilize them effectively (or even what they’re names are for that matter).

Recently, our team had the grand idea of trying to use two snowblower motors to control an arm on our robot to lift the portcullis and push down the cheval de frise (we wanted a motor with a form of worm lock to increase stability). After spending ten minutes or so on the forums I realized this was a terrible idea since we wouldn’t have had anything between the motor and the arm and probably ended up with two dead motors by the end of our first few practice sessions.

Since we have a low budget and limited knowledge, one of our team members suggested using our two CIMple Boxes from last year’s chassis to control our arm with 1 CIM on each box.

My two questions are:

  1. Is the above idea with 2 CIMple Boxes horrible and completely inefficient? Would it make due if we didn’t want to purchase any additional materials? Even thought this setup would require additional programming to maintain the arm’s angle, I’m fairly certain we’re capable.

  2. Can you please point me in the direction of an organized location where I can learn about the basics of motors and their various accessories designed with FRC materials in mind?

Thank you for taking the time to read about my team’s plight, any help we receive is always greatly appreciated!

**Again, thank you to everyone for the helpful responses! It never hurts to have more information!

#2

A cimplebox will not have nearly enough reduction, and will draw massive current from your motors.

At the very least, download JVN calculator. It will allow you to find good ratios for your designs even without knowing too much about motor selection (because that’s what you use it for).
I don’t know where to learn more in-depth stuff about motors though. The Vex motor curves are nice but do not tell you how to get those graphs, or how to interpret them.

#3

Read this web page and follow the links:

http://motors.vex.com/introduction

… then start asking specific questions here. You’ll get lots of help.

Good luck.

#4

As for learning how to calculate an ideal gear ratio for a drivetrain, this presentation was particularly helpful when I started learning drivetrain design:
http://first.wpi.edu/Images/CMS/First/2005CON_Advanced_Drivetrain_Calcs.ppt

Also check out Team 973’s RAMP video series. They have several videos on selecting motors for a number of applications (And many other topics as well):

https://www.youtube.com/user/973RAMP

#5

*Here’s a very simple motor-sizing example.

Suppose you want to lift a 25 pound weight 4 feet in 5 seconds.

To figure the power required to do that, you multiply 25 times 4, and divide by 5. That’s 20 foot pounds per second.

Multiply that answer by 1.356 to convert the answer to watts. That’s 27 watts.

That’s the absolute minimum mechanical power you will need. Increase that by, say, 50% to give you a safety margin. That’s 41 watts.

Now look at the spec sheet for the various motors and pick a motor (or an identical pair of motors*) whose max output mechanical power is greater than 41 watts.

One you’ve selected the motor(s), you can compute the appropriate gear ratio.

  • the power of an identical pair of motors is twice the power of one of those motors

#6

GameSense hosts a series in partnership with FIRST called Behind The Lines. We did an episode with Ken Stafford from WPI about this very subject.

#7

Are there any simple examples like this for anything besides lifting weights? I’ve heard this example before (and it’s a great example for helping understand how to pick motors for a lifting mechanism) but I was wondering if there were any simple ways of thinking about other mechanisms. Even just good rules of thumb from experience would be appreciated.

#8

LIke what? Pick one.

#9

The most important application of motors in FRC is the drivetrain. Since driving moves motors from Stall Torque to Free Speed repeatedly it is important to consider the Sprint Distance that you want to optimize. Here is a list of the other requirements your drivetrain should meet.

  • Sprint Speed should be optimized
  • Pushing Force should be optimized if you have a second gear
  • Current per Motor Channel should not be exceeded under sprint, wall-slip and turning conditions
  • Current per Main Breaker should not be exceeded under sprint, wall-slip and turning conditions
  • Bus Voltage Drop should not be excessive
  • Battery should support sustained current draw for duration of match.

Focusing on the sprint speed requirement you should consider what the most important traversal your robot will perform for this year’s particular game. Last year, FRC687 optimized for a 12 ft sprint distance since that was 2/3 of the distance to the far tote goal.

Then you should consider the motors you will allocate to your drive. In this case, you might choose between a six-CIM drive and four-CIM drive. The drivetrain calculators below are very powerful in that they simulate the acceleration of the robot during a sprint and can give you an accurate picture of the performance of your drivetrain at different gear ratios.

My drivetrain calculator, which borrows heavily from JVN and JesseK, allows you to input your desired drivetrain and identify the best gear ratio for that scenario directly by reading the graph of time-to-distance vs. gear ratio. You can access it below.

JVN

JesseK

FRC687

#10

BTW, Ether, thanks for all of your thoughtful posts. Particularly the ones on mini-bots for Logomotion.

#11

A couple I can think of:

Figuring out how much power you need to push an object.

Flywheel shooters (I doubt there’s anything basic here since there are so many variables).

#12

Let W be the weight of the object (Newtons)

Let mu be the kinetic friction coefficient between the object and the floor (unitless)

Let V be the desired pushing speed (meters/sec)

Required power P = WmuV watts

Let E be the power efficiency of your drivetrain

Total Motor output mechanical power M = P/E watts

Total Motor electrical power consumption can then be estimated using motor curves

#13

You don’t necessarily have to use identical motors. It does however keep the calculations a bit simpler, which is a good recomenfldation for beginners.

#14

“Beginners” definitely was the context here.

Here’s a thread about combining unmatched motors.